Answer
The solutions are $a=6$ or $a=1$.
Work Step by Step
$a=\sqrt 7a-6$
Square both sides:
$(\sqrt 7a-6)^2=a^2$
$a^2-7a+6=0$
$(a-6)(a-1)=0$
$a=6$ or $a=1$
With $a=6$
then $a=\sqrt 7a-6$
$6=\sqrt 7(6)-6$
$6=6$
With $a=1$
then $a=\sqrt 7a-6$
$1=\sqrt 7(1)-6$
$1=1$
Hence, the solutions are $a=6$ or $a=1$.